This invention relates generally to apparatus for use with natural gas dehydrators of the type used to remove water and water vapor from a natural gas stream composed of a mixture of natural gas, liquid hydrocarbons, liquid hydrocarbon vapors, water and water vapors and is particularly directed to apparatus for use with field natural gas dehydrators.
An example of such a field natural gas dehydrator is disclosed in U.S. Pat. No. 5,766,313 to Rodney Thomas Heath and the disclosure therein is specifically incorporated herein by reference thereto. In general, such systems comprise a separator means for receiving the oil and water liquids from xe2x80x9cwetxe2x80x9d (water vapor laden) gas; and a water absorber means, which employs a liquid dehydrating agent such as glycol, for removing the water vapor from the wet gas and producing xe2x80x9cdryxe2x80x9d gas suitable for commercial usage. The glycol is continuously supplied by a pump to the absorber means in a xe2x80x9cdryxe2x80x9d low-water vapor-pressure condition and is removed from the absorber means in a xe2x80x9cwetxe2x80x9d high-water vapor-pressure condition. The wet glycol is continuously removed from the absorber means and circulated through a reboiler means, which includes a still column, for removing the absorbed water from the glycol and heating the glycol to provide a new supply of hot dry glycol. Heating of the glycol in the reboiler means is generally accomplished through use of a gas burner mounted in a fire tube. The hot dry glycol from the reboiler means passes through a heat exchanger, where the hot dry glycol transfers some of its heat to incoming wet glycol going to the still column. The dry glycol subsequently passes to a dry glycol storage tank. A glycol passage means is provided to enable passage of wet glycol from the absorber means to the reboiler means and to pump dry glycol from the storage tank to the absorber means. Besides water, the wet glycol going to the still column of the reboiler of the natural gas dehydrator will contain natural gas and absorbed hydrocarbons.
On many dehydrators, a volume of natural gas is intentionally induced into the reboiler in order to dry the wet glycol to a higher concentration than can be accomplished by simply adding heat. The process of intentionally inducing a volume of natural gas into the reboiler is referred to as gas stripping.
In the still column of the reboiler of the natural gas dehydrator, the water, natural gas, and other hydrocarbons are separated from the glycol by the pressure reduction from the absorber pressure to approximately atmospheric pressure in the still column and by the application of heat from the burner in the fire tube of the reboiler.
The water, natural gas, and other hydrocarbons contained in the wet glycol stream which are separated in the still column from the wet glycol will be exhausted into the atmosphere through the atmospheric vent on the still column. The hydrocarbon vapors released through the still column of a natural gas dehydrator are air pollutants. Specifically, certain hydrocarbons such as benzene, toluene, ethylbenzene, and xylene, commonly referred to as BTEX have been proven to be carcinogenic.
The gas dehydrator disclosed in U.S. Pat. No. 5,766,313 offers solutions to at least some of the problems discussed above but improvements can be made to such a gas dehydrator.
This invention provides apparatus for use with a field natural gas dehydrator in which the wet glycol from the absorber is fed into a three phase emissions separator apparatus which is provided with suitable apparatus, such as an overflow tank, that contains an amount of wet glycol, part of which is used as a circulating medium as described more fully below, and routes another part of the wet glycol, such as an amount equal to the amount of wet glycol received from the absorber, to a reboiler. In the description of the invention, the apparatus is described in its operating condition. The circulating wet glycol is used to cool the emissions from the still column of a reboiler and to provide the energy to create a vacuum using an eductor as described more fully below. By using the part of the wet glycol as the pump circulating medium, the lubricity, vapor pressure, viscosity, and etc. of the circulating medium remain relatively constant from dehydrator to dehydrator; therefore overcoming potential pump problems which could occur if a circulating medium with changing physical constants, from dehydrator to dehydrator, was used.
Also, the wet glycol can be pumped in greater volumes and used as a heat exchange medium for condensing the effluent from the still column of the reboiler. As explained more fully below, the volume of the wet glycol being pumped by the circulating pump is 20 to 30 times greater than the volume of the wet glycol exiting the absorber.
In one embodiment, this invention also provides a liquid water separator and removal apparatus that collects the liquid water condensed from the effluent from the still column so that substantially no water in the form of vaporized water or liquid water is returned into the emissions separator. Ideally, substantially all of the water in the vaporized water in the effluent is changed into liquid water in an effluent condenser apparatus and is fed into and removed in the liquid water separator and removal apparatus. This is accomplished in the instant invention by cooling the effluent to a temperature in the effluent condenser that condenses substantially all of the vaporized water and even some of the vaporized hydrocarbons to produce liquid hydrocarbons.
The invention also provides a system for collecting the gases from the level controllers used in the natural gas dehydrator so that the collected gases may be used as fuel for the burner of the reboiler.
In accordance with one embodiment of this invention, a natural gas dehydrator is provided wherein a supply of natural gas is fed into an absorber wherein it is subjected to dry glycol to remove undesirable materials therefrom so that the dry glycol is changed into wet glycol that is removed from the absorber and fed at reduced pressure into a first separator comprising a three phase emissions separator apparatus. When the invention is under operating conditions, the three phase emissions separator apparatus has liquid level control apparatus that provides for holding a predetermined amount of wet glycol some of which is to be circulated as described below and some of which wet glycol that is transferred to the still column of a reboiler apparatus that receives the wet glycol and changes such amount of wet glycol into dry glycol and vaporized effluent. Under operating conditions, the amount of wet glycol being transferred to the reboiler is substantially equal to the amount of wet glycol being fed into the three phase emissions separator apparatus from the absorber. An effluent condenser apparatus is provided for receiving the vaporized effluent. The effluent condenser apparatus comprises a hollow shell having a tube extending therethrough wherein the tube has an external spiral fin. The hollow shell has an inlet and an outlet so that the wet glycol can be circulated therethrough. The effluent from the still column is fed into the tube and is cooled by the circulating wet glycol. Circulating apparatus is provided for circulating wet glycol from the three phase emissions separator apparatus through the effluent condenser apparatus to change the vaporized effluent to at least liquid water, liquid hydrocarbons and uncondensed vapors, which are substantially hydrocarbon vapors, and returning the circulating wet glycol with other ingredients entrained therein to the three phase emissions separator apparatus. Second separator apparatus comprising liquid water separator and removal apparatus is provided for receiving the at least liquid water, liquid hydrocarbons and uncondensed vapors for separating and removing the liquid water. Additional apparatus is provided for removing the liquid hydrocarbons and the uncondensed vapors from the liquid water separator and removal apparatus and feeding the liquid hydrocarbons and the uncondensed vapors to the three phase emissions separator apparatus.
The additional apparatus comprises an eductor having an inlet port, an exit port and a vacuum port. A first conduit through which the circulating wet glycol flows is connected to the inlet port. A second conduit extends between the outlet port and the three phase emissions separator apparatus. A third conduit extends between the liquid water separator and removal apparatus and the vacuum port of the eductor to form at least a relative vacuum in the liquid water separator and removal apparatus.
The liquid water separator and removal apparatus comprises a hollow shell having a partition therein for forming at least a first and a second chamber in the hollow shell. A first outlet port is formed in the first chamber and is connected to the third conduit so that a vacuum is formed in the first chamber. A first inlet port is formed in the first chamber so that the at least liquid water, liquid hydrocarbons and uncondensed vapors from the condenser apparatus can flow into the first chamber and be separated into at least an upper layer comprising the uncondensed vapors, a middle layer comprising the liquid hydrocarbons and a lower layer comprising the liquid water. The first outlet port is located so that the uncondensed vapors and the liquid hydrocarbons can flow through the first outlet port into the third conduit. The withdrawn uncondensed vapors and the liquid hydrocarbons enter the eductor and are compressed and entrained into the wet glycol and flow with the wet glycol from the eductor to the three phase emissions separator. A second outlet port is formed in the first chamber and a second inlet port is formed in the second chamber. A conduit connects the second outlet port and the second inlet port so that the liquid water can flow from the first chamber into the second chamber. A drain port is formed in the second chamber for draining liquid water from the second chamber.
The natural gas dehydrator has at least one gas emitting level control apparatus. Each of the three phase emissions separator apparatus and the liquid water separator and removal apparatus has a gas emitting level control apparatus. A gas inlet port is formed in the liquid water separator and removal apparatus. Collecting apparatus is provided for collecting the gases emitted from the gas emitting level control apparatus and conduits extend between the collecting apparatus and the gas inlet port for transmitting the gases to the gas inlet port.
In the operation of the above-described apparatus a supply of natural gas is fed into an absorber wherein it is subjected to dry glycol to remove undesirable materials therefrom so that the dry glycol is changed into wet glycol that is removed from the absorber and is processed by feeding the wet glycol from the absorber into the three phase emissions separator apparatus; retaining a supply of wet glycol to a predetermined level in the three phase emissions separator apparatus; feeding wet glycol greater than the predetermined level from the three phase emissions separator apparatus to the still column of a reboiler for changing such wet glycol into dry glycol and vaporized effluent; feeding the vaporized effluent to a condenser apparatus; circulating wet glycol into and out of the retained wet glycol in the three phase emissions separator apparatus so that wet glycol flows through the condenser apparatus to change the vaporized effluent to at least liquid water, liquid hydrocarbons and uncondensed vapors and returns the circulating wet glycol with other added materials to the three phase emissions separator apparatus; feeding the at least liquid water, liquid hydrocarbons and uncondensed vapors to a liquid water separator and removal apparatus; separating and removing the liquid water from the at least liquid water, liquid hydrocarbons and uncondensed vapors; draining the removed liquid water; and entraining the at least liquid hydrocarbons and the uncondensed vapors into the circulating wet glycol to be returned to the three phase emissions separator apparatus.
A vacuum is formed in the liquid water separator and removal apparatus by positioning an eductor having an inlet port, an outlet port and a vacuum port between the liquid water separator and removal apparatus and the three phase emissions separator apparatus; feeding the circulating wet glycol to the inlet port; passing the circulating wet glycol through the eductor and out of the outlet port to create a vacuum to draw the uncondensed vapors and liquid hydrocarbons from the liquid water separator and removal apparatus; compressing the uncondensed vapors and entraining them with any liquid hydrocarbons into the circulating wet glycol; and feeding the circulating wet glycol with the entrained condensed vapors and liquid hydrocarbons from the outlet port into the three phase emissions separator apparatus.
The removal of the liquid water is accomplished by forming a first and a second chamber in the liquid water separator and removal apparatus; feeding the at least liquid water, liquid hydrocarbons and uncondensed vapors into the first chamber; separating the at least liquid water, the liquid hydrocarbons and uncondensed vapors in the first chamber; removing the liquid hydrocarbons and the uncondensed vapors from the first chamber; entraining the removed uncondensed vapors and liquid hydrocarbons into the wet glycol in the eductor; transferring at least a portion of the liquid water from the first chamber to the second chamber until the liquid water in the second chamber reaches a predetermined level; and removing at least a portion of the liquid water from the second chamber.
Additional gas is transferred to the first chamber by providing at least one gas emitting level control apparatus in at least the absorber, the three phase emissions separator apparatus and the liquid water separator and removal apparatus; collecting the gases emitted by the gas emitting level control apparatus; and feeding the collected gases into the first chamber.
In another embodiment of the effluent condenser apparatus, the finned tube is connected to the effluent piping carrying the effluent issuing from the still column and is located within the hollow shell as described above. Also, the hollow shell has longitudinally extending external fins. The modified effluent condenser apparatus is located within a hollow tube having a diameter greater than the external fins of the hollow shell. The hollow tube has an enlarged cross section at one end wherein a fan is located to blow air over the finned hollow shell. Also, a portion of the tubing through which the cooled effluent passes is exposed to the air. A thermostat is located in the piping or line carrying the cooled effluent comprising hydrocarbon gas, liquid hydrocarbon, water and water vapors from the effluent condenser apparatus to the liquid separator and removal apparatus. The fan is turned on or off in response to the temperature in the cooled effluent as described below.
In a further embodiment of the effluent condenser apparatus, the wet glycol is not passed through the hollow shell. In this embodiment, only the finned tubing is located within the hollow tube having the fan located therein. A thermostat is also located in the line carrying the cooled effluent from the effluent condenser to the liquid separator and removal apparatus and turns the fan on or off in response to the sensed temperature.
In another embodiment of the invention, a two phase emission separator apparatus is used instead of the three phase emission separator apparatus and such two phase emission separator apparatus is described more fully below.
In another embodiment of the invention, a modified liquid separator and removal apparatus acts on the cooled effluent to separate and remove both the liquid water and the liquid hydrocarbons from the cooled effluent leaving at least uncondensed hydrocarbon gases which are drawn from the liquid separator and removal apparatus by the vacuum formed by the eductor as described above.
This modified liquid separator and removal apparatus comprises a hollow shell having two sealed partitions therein for dividing the hollow shell into first, second and third chambers. The first chamber receives the cooled effluent comprising at least liquid water, liquid hydrocarbons and uncondensed vapors. The first chamber has a first outlet port through which the uncondensed vapors, having at least hydrocarbon gases, are drawn into the eductor by the vacuum therein. A second outlet port in the first chamber removes the liquid hydrocarbons from the first chamber and deposits the liquid hydrocarbons in the second chamber from which they are removed as described below. A third outlet port in the first chamber removes the liquid water from the first chamber and deposits the liquid water into the third chamber from which they are removed as described below.
The invention also provides a heat exchanger for heating the wet glycol flowing from the two or three phase emission separator. Wet glycol is moved from the two or three phase emission separator through a heat exchanger coil within the storage tank for the hot dry glycol received from the reboiler whereby the temperature of such wet glycol is raised. This higher temperature processed wet glycol is then passed through an outer shell of a heat exchanger which has a finned tubing encased therein. The circulating wet glycol from the two or three phase emissions separator apparatus passes through the finned tubing so that the temperature of the circulating wet glycol therein is raised. The passage of the circulating wet glycol through the finned tubing in the heat exchanger is controlled by a thermostat associated with the two or three phase emissions separator apparatus.
The operation of the invention using the wet glycol heat exchanger, the effluent condenser apparatus wherein the wet glycol is used to treat the effluent from the still column and the modified liquid separator and removal apparatus is as follows. The effluent issuing from the still column is fed into the finned tubing in the effluent condenser apparatus. The wet glycol flows in the hollow shell around the hot effluent in the finned tubing to produce a treated effluent comprising at least liquid hydrocarbons, liquid water and uncondensed vapors containing at least gaseous hydrocarbons all of which pass out of the effluent condenser apparatus and into the modified liquid separator and removal apparatus. The thermostat senses the temperature of the circulating wet glycol and determines whether or not the circulating wet glycol from the emissions separator apparatus is or is not to be passed through the heat exchanger by the operation of a valve controlled by the thermostat. When the valve is fully closed, the circulating wet glycol is fed to the effluent condenser apparatus. When the valve is fully open, the circulating wet glycol is fed through the heat exchanger and the liquid separator and removal apparatus to the eductor. As the valve moves between the fully opened and fully closed positions, the amount of the circulating wet glycol passed through the heat exchanger is proportionally changed. In some instances, it may be necessary to use the fan to further cool the effluent passing through the pipe in the effluent condenser apparatus. The treated effluent enters the first chamber of the modified liquid separator and removal apparatus wherein the uncondensed vapors are removed by the vacuum in the eductor; the liquid hydrocarbons are removed from the first chamber to the second chamber and the liquid water is removed from the first chamber to the third chamber. When the liquid hydrocarbons in the second chamber reach a predetermined level, at least a portion of the liquid hydrocarbons in the second chamber are removed therefrom. When the liquid water in the third chamber reaches a predetermined level, at least a portion of the liquid water is removed therefrom. The circulating wet glycol passes through the eductor to create the above-described vacuum and the uncondensed vapors from the modified liquid separator and removal apparatus are compressed and entrained in the circulating wet glycol and then pass into the emissions separator apparatus.
The operation of the invention not using the circulating wet glycol in the effluent condenser apparatus but using the heat exchanger and the modified liquid separator and removal apparatus is as follows. The hot effluent from the still column flows into the finned tubing in the effluent condenser apparatus. A fan, controlled by another thermostat, operates to cool the hot effluent to produce at least liquid hydrocarbons, liquid water and uncondensed vapors containing at least gaseous hydrocarbons. If the outside temperature is low enough, it is generally not necessary to operate the fan to cool the hot effluent. Also, the thermostat in the emissions separator apparatus or tubing or piping functions to determine whether or not the circulating wet glycol from the emissions separator apparatus is passed through the heat exchanger. When the valve is fully closed, the circulating wet glycol is fed through a heated coil in the liquid separator and removal apparatus and then to the eductor. When the valve is fully opened, the circulating wet glycol is fed through the heat exchanger and then directly to the eductor. The treated effluent flows into the modified liquid separator and removal apparatus and is processed thereafter as described above.
In another embodiment of the invention, prevention apparatus is provided to prevent the inadvertent transfer of liquid hydrocarbons to the reboiler and in particular from the two phase emissions separator apparatus. The build up of liquid hydrocarbons in a two or three phase emissions separator apparatus can result from the liquid hydrocarbons in the wet glycol from the absorber, the carryover from the liquid separator and removal apparatus and the further condensation of the hydrocarbon gases in the two or three phase emissions separator apparatus. The prevention apparatus is described preferably in relation to a two phase emissions separator apparatus and comprises a throttling liquid level control apparatus that is set to control the level of the total liquids in the emissions separator apparatus which total level comprises liquid hydrocarbons and wet glycol. A float that is weighted to float on top of the wet glycol is connected to apparatus for opening or closing a solenoid valve for purposes described below. A casing having a closed lower end and an open upper end is located at a preset location in the emissions separator apparatus. A pipe having an open bottom end is located within the casing and its open bottom is located about one inch from the closed lower end and has its other open top end connected to the solenoid valve. The emissions separator apparatus has a lower level of wet glycol, an intermediate level of liquid hydrocarbons and an upper level of gaseous hydrocarbons. An open ended tube has its upper open end located in the emissions separator apparatus so that it is in the gaseous hydrocarbon level and its lower open end located adjacent to but spaced from the bottom of the emissions separator apparatus so that it is located in the wet glycol level. An open ended pipe has a portion thereof located in the open ended tube so that its lower open end is located to be exposed under normal conditions to the wet glycol. The upper open end of the open ended pipe is secured to a dumping apparatus for transferring wet glycol from the emissions separator apparatus to the still column.
In the operation of the prevention apparatus, when there is only a relatively small amount of liquid hydrocarbons in the emissions separator apparatus, the float on the wet glycol will close the solenoid valve and the throttling liquid level apparatus will operate to dump wet glycol to the still column of the reboiler as the wet glycol from the absorber enters the emissions separator apparatus. When the level of the liquid hydrocarbons reaches a predetermined amount, the level of the wet glycol will reach a lower level at which time the float will open the solenoid valve so that the liquid hydrocarbons will flow through the solenoid valve since the emissions separator apparatus is at a pressure higher than atmospheric. This causes a drop in the total liquid level in the emissions separator apparatus so that the throttling liquid level control apparatus shuts off the dumping apparatus so that no wet glycol is transferred to the still column of the reboiler. Since the wet glycol from the absorber continues to enter the emissions separator apparatus, the level of the wet glycol will start to rise in the emissions separator apparatus. Since the liquid hydrocarbons are dumped at a rate greater than the rate of entry into the emissions separator apparatus of the wet glycol from the absorber, a substantial amount of the liquid hydrocarbons will be dumped before the level of the wet glycol in the emissions separator apparatus raises the weighted float to a level to close the solenoid valve. When the total level of liquid hydrocarbons and wet glycol is detected by the throttling liquid level control apparatus, wet glycol from the emissions separator apparatus will again be dumped to the reboiler.